This invention relates to color measuring instruments and, in particular, to methods and apparatus for calibrating and verifying the calibration of color measuring instruments that are suitable for measuring the color of a surface, including human skin.
It is well known that color measuring instruments, such as colorimeters and spectrophotometers, can be used to measure the color of surfaces for a variety of useful applications. For example, Macfarlane et al. U.S. Pat. No. 5,313,267 describes a method and instrument for selecting personal compatible colors using a color measuring device. Also, PCT Publication No. WO 96/41140 describes methods and apparatus for determining the condition of a test subject based on color by using a color measuring instrument to detect change in a color factor indicative of a condition such as a disease, spoilage, ageing, etc. In particular, those methods and apparatus can be used to detect a medical condition known as hyperbilirubinemia by measuring skin color.
However, in order to ensure that the skin color measurements are accurate, they are usually preceded by conventional calibration and optionally conventional verification techniques. Such calibration can be performed by measuring a single color standard of predetermined coloration having premeasured color measurement value(s) and comparing the measurement value(s) with the premeasured value(s) to see that they are substantially the same. Once calibrated, the instrument is normally prepared to perform accurate color measurements producing color measurement value(s) that characterize the color of an unknown sample.
Additionally, further subsequent verification against a color standard that is different than that of the color standard used during calibration can help to ensure that the standard used to calibrate the color measuring instrument was an accurate and authorized color standard for its intended calibration and that calibration was performed according to predetermined specifications.
When color measuring instruments are used for medical applications, such as when they are used to detect abnormal medical conditions (including hyperbilirubinemia), the accuracy of the color standard used for calibration and verification is critical. However, color standards may be inaccurate because they have been inadvertently scratched and/or soiled by previous users or because they have become discolored, or faded, from atmospheric or electromagnetic radiation exposure. Furthermore, another reason for inaccuracy of the color standard includes poor quality control at the site where the color standard is manufactured. Poor quality control at the manufacturing site may yield unacceptable variability in coloration of a color standard (even though the color standard is new and unused). Moreover, color standards from unauthorized manufacturing sites may be mislabeled.
Furthermore, when a color measuring instrument is used to determine a medical condition, a portion of the instrument usually makes an optical seal on the surface of the patient's skin. In order to make the seal, however, that portion of the color measuring instrument normally makes physical contact with the skin. A disadvantage of such contact is that the contacting surface of the instrument could transfer germs between successive patients when a single color measuring instrument is used to make such measurements.
In view of the foregoing, it is an object of this invention to provide sanitary methods and apparatus for making color measurements of human skin among other uses for such an invention, such as measuring laboratory specimens.
It is yet another object of this invention to provide improved color calibration and verification methods and apparatus that can detect inaccurate or unauthorized color standards and inaccurate calibration of color measuring instruments.
It is another object of this invention to provide methods and apparatus for safely measuring medical conditions without the risk of germ transfer.